Tetrafluoroethylene (TFE) polymers are well known. The group of TFE polymers includes polytetrafluoroethylene (PTFE), which was originally disclosed by Plunkett in U.S. Pat. No. 2,230,654, and copolymers of TFE with such small concentrations of copolymerizable modifying monomers that the melting point of the resultant polymer is not substantially reduced below that of PTFE, e.g., no lower than 320.degree. C., (modified PTFE). The modifying monomer can be, for example, hexafluoropropylene (HFP), perfluoro(propyl vinyl ether) (PPVE), perfluoro(ethyl vinyl ether) (PEVE), chlorotrifluoroethylene (CTFE), or other monomer that introduces bulky side groups into the molecule. The concentration of such modifiers is usually less than 1 wt %, more commonly less than 0.5 wt %. Various processes for polymerizing TFE have evolved since Plunkett's discovery.
PTFE and modified PTFE can be produced by the process known as dispersion polymerization, which typically yields an aqueous dispersion (raw dispersion) of small particles which can be coagulated and dried to obtain coagulated dispersion resin (also known in the art as fine powder) or concentrated and/or stabilized for use as a dispersion. The dispersion polymerization of TFE in an agitated aqueous medium using fluorinated surfactant (dispersant) and water-soluble initiator is well known, dating from U.S. Pat. No. 2,559,752 (Berry). The process is usually carried out in the presence of 0.1-12%, based on weight of aqueous medium, of a saturated hydrocarbon having more than 12 carbon atoms and which is liquid under polymerization conditions, as disclosed by Bankoff in U.S. Pat. No. 2,612,484. Paraffin wax is a preferred saturated hydrocarbon, which sets a lower limit on polymerization temperature for the paraffin to be present in its liquid state. Paraffin waxes commonly used for this purpose melt at about 50-60.degree. C., so dispersion polymerizations commonly start at temperatures above about 60.degree. C. Such hydrocarbons act as stabilizers in the polymerization process, preventing or retarding the formation of coagulated polymer in the agitated system.
Fine powder resin, whether PTFE or modified PTFE, has high melt viscosity, e.g. a melt viscosity of at least 1.times.10.sup.8 Pa.multidot.s. Such resin does not flow readily at melt temperature and, therefore, is considered to be non-melt-fabricable. Fine powder resin is commonly converted to useful articles by a lubricated extrusion (paste extrusion) process in which the resin is blended with a lubricant, the lubricated resin (paste) is shaped by an extrusion process, the lubricant is removed, and the resultant green shape is fused (sintered) at temperature above the melting point of the PTFE.
One important use of fine powder resin has been to provide paste extruded shapes that can be rapidly stretched in the unsintered state to form product that is porous to water vapor but not to condensed water, and is useful in "breathable" fabric material for garments, tenting, separatory membranes, and the like. Other important uses for fine powder resins, including modified PTFE resins, include paste extruded wire insulation and tubing that is not stretched.
In practice, fine powder resin which has achieved acceptance for stretching use has been PTFE of high molecular weight. Resin for this utility is disclosed, for example, by Holmes in U.S. Pat. No. 4,016,345 which claims a process using inorganic persulfate initiator at a temperature of 95-125.degree. C. The '345 patent demonstrates stretching at the rate of 100%/sec. Koizumi et al. in U.S. Pat. No. 4,159,370 disclose a stretchable PTFE fine powder having molecular weight of not less than 5,000,000 and a process therefor using persulfate initiator in which the polymerization conditions are changed after initiation of polymerization. One of the alternative changes of conditions disclosed is lowering the polymerization temperature by 5-30.degree. C. Resins of the '370 patent examples are stretchable at 100%/sec. Shimizu & Koizumi in U.S. Pat. No. 4,363,900 disclose a dispersion polymerization process for preparing stretchable fine powder comprising incorporating into the aqueous medium at a specified point in the process a polymerization retarder, e.g., hydroquinone, to extend the polymerization time by at least 130%. This patent characterizes the PTFE fine powder of Koizumi et al. as having good stretchability but still somewhat difficult to attain uniform stretching. The '900 patent examples show stretching at 100%/sec to a draw ratio of 30 without breaks ("cuts") and with uniformity ranging from even to uneven.
Malhotra advanced the art of stretchable PTFE fine powder in U.S. Pat. No. 4,576,869 and U.S. Pat. No. 4,654,406 wherein the achievements of the resins disclosed therein include uniformity of stretch of at least 75% (i.e., good stretching uniformity) for stretching by at least 1000% at stretch rates throughout the range from 10%/sec to 100%/sec at a lubricant loading of 17 wt %. The superiority of Malhotra's PTFE is signaled by its capability for uniform stretching at the very low 10%/sec rate. The superior performance of the '869 patent resin was attained by ceasing to add permanganate initiator toward the end of the batch so that the reaction slows down and the end point is at least 5% longer in comparison with a reaction in which initiator addition is continued to the end of the reaction.
Despite the success of the Malhotra resin in stretching applications, improvements in stretched product characteristics, e.g., strength, are desired, as are improvements in polymerization productivity.
Shimizu et al. in U.S. Pat. No. 4,098,975 disclose a dispersion process for polymerizing TFE wherein at least 30% of the TFE to be polymerized is polymerized at one temperature and further polymerization is at a different temperature, the resultant PTFE having an amorphous index at least 0.01 higher than if the entire polymerization were conducted at the first temperature. While 30.degree. C. is mentioned as an example of second stage polymerization temperature, the reaction is said to require the use of a stabilizer melting below 30.degree. C. The polymerization is said to be free of any practical problems when temperatures are 70.degree. C. in the first stage and 90.degree. C. in the second stage, and examples of the invention all use 70.degree. C. in the first stage.
Herisson in U.S. Pat. No. 4,481,343 discloses a dispersion process for polymerizing TFE without a stabilizer, in which the polymerization starts at a temperature in the range 0-45.degree. C. and follows an ascending curve, the initiator yielding free radicals in the range 0-45.degree. C. and being destroyed for a temperature above 45.degree. C. Although advantages are recited by Herisson, including high molecular weight, the fact of not polymerizing at a constant temperature is considered to be unfavorable (column 2, lines 64-65). EXAMPLE 5 is a comparison to show the detrimental effect of a stabilizer of the paraffin or saturated liquid hydrocarbon type. However, for sustained manufacturing of PTFE resins by the dispersion polymerization process, a stabilizer of the saturated liquid hydrocarbon type is necessary to prevent accumulation of coagulum over successive batches in commercial operation.